1. Field of the Invention
The invention relates to an MR device, including
a) a magnetic field device for generating a uniform, steady magnetic field in an examination zone, PA1 b) an RF system for generating magnetic RF pulses in the examination zone, PA1 c) a reference coil system for receiving MR signals from an object present in the examination zone, PA1 d) a processing unit for forming MR images from the image data, derived from the MR signals of the coil elements of a array coil system, while taking into account reference data derived from the MR signals of the reference coil system.
2. Description of Related Art
An MR device of this kind is known from EP-A-0 695 947 which corresponds to U.S. Pat. No. 5,600,244. A coil array system described therein is available from Philips Medical systems under the trademark "SYNERGY". The coil elements of the coil array system (usually surface coils) receive the MR signals from the object areas in the vicinity of the coils with an improved signal-to-noise ratio (in comparison with a whole-body coil). However, the sensitivity of the individual coil elements is highly location-dependent; the sensitivity is also dependent on the position of the individual coil elements in relation to the object to be examined (the location-dependent sensitivity of an MR coil is defined as the quotient of the magnetic induction in a location and the current in the coil which causes this magnetic induction at this location). Therefore, the data generated by the coil array system must be corrected so as to compensate for the location-dependent differences in sensitivity.
In the known device this correction is performed by means of the reference coil system which is constructed as a whole-body coil and whose sensitivity is substantially independent of location and also independent of the object present in the examination zone. Using the reference data derived from the MR signals received by the reference coil system, the image data supplied by the coil elements of the coil array system are corrected so that an MR image can be reconstructed which reproduces the object to be examined with an enhanced signal-to-noise ratio and a locally more uniform sensitivity.
It is a drawback of the known MR device that the measurements by means of the coil array system on the one hand and the reference coil system on the other hand must be performed separately in time, because an inductive coupling exists between these two coil systems; this inductive coupling has a significant effect on the signal-to-noise ratio of the MR signals received. The measuring period during which the two coil systems acquire the MR signals is thus prolonged.